A variety of reinforcing schemes have been used historically to reinforce irregular rock structures, and particularly those irregular rock structures encountered in mining operations. Common strategies have included the use of wooden boards to reinforce earthen tunnels, or installing an assembly of bolts and wire mesh and/or applying cement on the exposed surfaces of an excavation. More recently, various polymeric materials have come into use as reinforcing coatings.
U.S. Pat. No. 4,046,357 describes an apparatus and method for the application of a fast drying, pumpable cement to the walls and ceilings of mine tunnels and shafts. However, there are several limitations related to using cementitious technology, including the large equipment required for mixing and dispensing, messy application and the resulting time and effort related to cleaning up after application, and drying time required (from 15 minutes to over four hours, depending on the coating and atmospheric conditions). In confined regions of mines having ceiling heights of no more than 4 to 5 meters, or even 1.2 to 1.5 meters, it may be preferable to use smaller, lighter weight resin-dispensing systems. Another disadvantage of using cement for reinforcement is the low elongation or ability to respond to rock movement.
U.S. Pat. No. 4,715,746 describes a method of strengthening geological formations in underground workings or mines by introducing mixtures that form polyurethanes, typically by introducing polyisocyanate and polyol components into holes bored for the purpose.
U.S. Pat. Nos. 4,738,989, 4,748,192, and 4,748,201 disclose aliphatic polyurethane compositions, including sprayable, fast-curing compositions.
U.S. Pat. No. 5,716,711 discloses a two layer structure for coating surfaces for providing a gas-tight, flexible fire retardant coating onto a concrete, masonry or rock surface which structure includes a layer of solvent-free, elastomeric unfoamed polyurethane having a tensile strength sufficient to prevent localized gravity falls of loose rock, concrete or masonry, and a layer of vermiculite bonded onto the polyurethane.
U.S. Pat. No. 6,780,459 discloses methods for stabilizing irregular structures such as rock structures in mines by providing a weather resistant and anti-corrosive polyurea coating.
U.S. Pat. No. 6,966,610 discloses a method for providing a protective coating in a mine comprising spraying onto the rock surfaces of the mine a hot melt adhesive in an amount to form a coating at least 1 mm thick and allowing the coating to solidify, thereby to provide a protective coating on the at least one rock surface of the mine.
U.S. Pat. No. 7,011,865, which is incorporated herein by reference, discloses an elastomeric polymeric film that can be used as a load-bearable coating, for example, to assist in protecting from rock bursts in a mine.